Disk drive memory systems have been used in computers for many years for storage of digital information. The digital information is recorded on concentric memory tracks of a magnetic disk medium. (The actual information is stored in the form of magnetic transitions within the medium.) The disks themselves are rotatably mounted on a spindle, and information is accessed by means of read/write heads generally located on pivoting arms which move radially over the surface of the disk. The read/write heads, or transducers, must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
During operation, the disks are rotated at very high speeds by means of an electric motor generally located inside the hub or below the spindle. One type of motor is commonly known as an "in-hub" or "in-spindle" motor. In this type of motor assembly the drive motor is incorporated within the spindle itself. Such a motor assembly is described in U.S. Pat. Nos. 4,754,353 and 4,928,029, both of which are assigned to the assignee of the present application. In-spindle motors have a spindle which is mounted by means of two ball-bearing systems to a non-rotating motor shaft disposed in the center of the hub. One of the bearings is typically located at the top of the spindle and the other is located at the bottom of the spindle. These bearings allow for rotational movement between the shaft and the hub while maintaining accurate alignment of the spindle to the shaft. The bearings themselves are normally lubricated by grease or oil, which creates a hydrodynamic bearing surface during operation.
The conventional bearing system described above is prone to several shortcomings. First, there is the problem of friction generated on the ball bearing surface. Since the ball bearings are always in mechanical or physical contact with either the grease lubricating layer or with the motor shaft itself, resulting friction limits the maximum speed of rotation that can be achieved. In other words, hydrodynamic mechanical bearings of this type are severely limited to a maximum rotational speed. This limitation conflicts with the need to spin the magnetic disk at ever higher speeds in order to improve the overall performance of the disk drive system.
Moreover, mechanical bearings are not always scaleable to smaller dimensions. This is a significant drawback since the trend in the disk drive industry has been to continually shrink the physical dimensions of the disk drive unit. Furthermore, the frictional nature of the ball-bearings leads to wear on the associated parts, thereby shortening the lifetime of the motor.
Another problem is that the seals which confine the grease and oil lubricants within the bearing are subject to leakage with increased wear. Any leakage into the disk drive environment is potentially hazardous since outgasing of contaminants can cause flaws and other data errors within the system.
In an attempt to overcome the problems associated with traditional ball-bearing systems, an alternative motor design has been developed that utilizes a dynamic pressure-type cylindrical groove bearing. This design, which is sometimes referred to as an "under-slung" motor, relies upon pressurized gas in the radial bearing gap between the spindle and motor shaft. This air pressure is increased by a series of dynamic pressure generating grooves located on the outer surface of the motor shaft. An example of this type of magnetic disk drive motor is disclosed in U.S. Pat. No. 4,656,545.
While the under-slung motor design benefits from the use of air pressure bearings, the design is not without its drawbacks. For example, such designs typically require that the entire disk drive assembly including the magnetic disks themselves be totally enclosed within a pressurized air chamber. This means that the drive assembly must be hermetically sealed, which is often expensive and difficult to implement.
In another attempt to overcome the problems of ball-bearing systems, a spindle motor design based on a gas dynamic bearing using double-sided grooves to generate dynamic pressure was developed as disclosed in U.S. Pat. No. 4,998,033. The bearing part of the motor is positioned in a manner that resembles either an under-slung or an over-hang. This requires extra space and is not suitable for small form-factor drives. The design uses a high viscosity lubricant and requires more electrical power to overcome increased drag force. It has only an unidirectional thrust mechanism and cannot work in all dispositions (horizontal, upside down, etc.). The design also has an insufficient mechanism for preloading and has no mechanism for providing radial balance at the top to control run-out.
What is needed then is an air-bearing system for a magnetic recording motor assembly which offers a simple construction, yet is capable of being manufactured at low cost and in high volume. As will be seen, the present invention provides a self acting air-bearing system that utilizes pressurized air as a bearing lubricant to overcome the problems inherent in the prior art. cl SUMMARY OF THE INVENTION
An air-bearing motor assembly for magnetic recording systems is disclosed. The primary advantages of using air as a bearing lubricant include a significantly lower coefficient of viscosity as compared to conventional hydrodynamic bearing systems. Hence, the power demand in the present invention is extremely low while the load carrying capacity remains essentially constant.
In one embodiment, the present invention comprises a motor assembly having a cylindrical shaft onto which is secured a stator. The stator has an electromagnetic coil disposed about the longitudinal axis of the shaft. Both the stator and the shaft are housed within a rotatable hub. A plurality of information recording magnetic disks are mounted in axially spaced apart relationship on the outer peripheral portion of the rotatable hub. Within the hub, a magnet means is attached and disposed radially about the stator for interacting electromagnetically with the coil to cause rotation of the hub relative to the shaft during normal operation.
The motor assembly also includes a pair of cylindrical sleeve members affixed to the hub and disposed about respective first and second ends of the shaft. Each of the sleeve members has a dry lubricated surface of a predetermined radial dimension which is calculated to be slightly larger than the radial dimension associated with the shaft. This creates a gap between the shaft and each of the sleeve members. Upon rotation of the hub relative to the shaft a pressurized air film develops within the gaps. It is these pressurized air films that function as the bearings for the system. The dimension of the gaps is preferably optimized based on the surface area of the bearings, the rotational speed of the hub, and the various loads that are supported. The gap-forming surfaces create axial force through pressure generation between the closely matched surfaces, and maintain the axial load and stiffness requirements.
The present invention is further characterized by its simple construction and its relatively low cost. Because the drive itself is normally in an air-ambient, there is no need to hermetically seal the motor assembly. Also, since air is chemically stable, the problem of outgasing into the disk drive environment is non-existent. In addition to being noiseless and vibration-free, the present invention is also characterized by a very long lifetime, since mechanical abrasion and wear are virtually eliminated.